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Date: Mon, 26 Oct 2015 18:08:59 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Vitaly Kuznetsov <vkuznets@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: Rasmus Villemoes <linux@...musvillemoes.dk>,
Ulf Hansson <ulf.hansson@...aro.org>,
James Bottomley <JBottomley@...n.com>,
Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] lib/string_helpers.c: don't lose precision in
string_get_size()
On Mon, 2015-10-26 at 14:55 +0100, Vitaly Kuznetsov wrote:
> string_get_size() loses precision when there is a remainder for
> blk_size / divisor[units] and size is big enough. E.g
> string_get_size(8192, 4096, STRING_UNITS_10, ...) returns "32.7 MB"
> while it is supposed to return "33.5 MB". For some artificial inputs
> the result can be ridiculously wrong, e.g.
> string_get_size(3000, 1900, STRING_UNITS_10, ...) returns "3.00 MB"
> when "5.70 MB" is expected.
>
> The issues comes from the fact than we through away
> blk_size / divisor[units] remainder when size is > exp. This can be
> fixed
> by saving it and doing some non-trivial calculations later to fix the
> error
> but that would make this function even more cumbersome. Slightly re-
> factor
> the function to not lose the precision for all inputs.
>
> The overall complexity of this function comes from the fact that size
> can
> be huge and we don't want to do size * blk_size as it can overflow.
> Do the
> math in two steps:
> 1) Reduce size to something < blk_size * divisor[units]
> 2) Multiply the result (and the remainder) by blk_size and do final
> calculations.
>
> Reported-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
> lib/string_helpers.c | 46 +++++++++++++++++++++++++-----------------
> ----
> 1 file changed, 25 insertions(+), 21 deletions(-)
>
> diff --git a/lib/string_helpers.c b/lib/string_helpers.c
> index f6c27dc..0658994 100644
> --- a/lib/string_helpers.c
> +++ b/lib/string_helpers.c
> @@ -43,41 +43,43 @@ void string_get_size(u64 size, u32 blk_size,
> const enum string_size_units units,
> [STRING_UNITS_10] = 1000,
> [STRING_UNITS_2] = 1024,
> };
> - int i, j;
> - u32 remainder = 0, sf_cap, exp;
> + int order = 0, j;
> + u64 remainder = 0;
> + u32 sf_cap;
> char tmp[8];
> const char *unit;
>
> tmp[0] = '\0';
> - i = 0;
Maybe leave i naming as is. Your order is not strictly speaking an
order, rather 3x order. I will make patch neater.
> if (!size)
> goto out;
>
> - while (blk_size >= divisor[units]) {
> - remainder = do_div(blk_size, divisor[units]);
> - i++;
> - }
> -
> - exp = divisor[units] / blk_size;
> /*
> - * size must be strictly greater than exp here to ensure
> that remainder
> - * is greater than divisor[units] coming out of the if
> below.
> + * size can be huge and doing size * blk_size right away can
> overflow.
> + * As a first step reduce huge size to something less than
> + * blk_size * divisor[units].
> */
> - if (size > exp) {
> + while (size > (u64)blk_size * divisor[units]) {
> remainder = do_div(size, divisor[units]);
> - remainder *= blk_size;
> - i++;
> - } else {
> - remainder *= size;
> + order++;
> }
>
> + /* Now we're OK with doing size * blk_size, it won't
> overflow. */
> size *= blk_size;
> + remainder *= blk_size;
> + /*
> + * We were doing partial multiplication by blk_size.
> + * remainder >= divisor[units] here means size should be
> increased.
> + */
> size += remainder / divisor[units];
> - remainder %= divisor[units];
> + remainder -= (remainder / divisor[units]) * divisor[units];
>
> + /*
> + * Normalize. size >= divisor[units] means we still have
> enough
> + * precision and dropping remainder is fine.
> + */
> while (size >= divisor[units]) {
> remainder = do_div(size, divisor[units]);
> - i++;
> + order++;
> }
>
> sf_cap = size;
> @@ -87,16 +89,18 @@ void string_get_size(u64 size, u32 blk_size,
> const enum string_size_units units,
> if (j) {
> remainder *= 1000;
> remainder /= divisor[units];
> - snprintf(tmp, sizeof(tmp), ".%03u", remainder);
> + /* remainder is < divisor[units] here, (u32) is
> legit */
> + snprintf(tmp, sizeof(tmp), ".%03u", (u32)remainder);
> tmp[j+1] = '\0';
> }
>
> out:
> - if (i >= ARRAY_SIZE(units_2))
> + if (order >= ARRAY_SIZE(units_2))
> unit = "UNK";
> else
> - unit = units_str[units][i];
> + unit = units_str[units][order];
>
> + /* size is < divisor[units] here, (u32) is legit */
> snprintf(buf, len, "%u%s %s", (u32)size,
> tmp, unit);
> }
--
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Intel Finland Oy
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